Test by Bikram | Databases | Test 1 | Question: 3

Question

Suppose that the transaction T1 and T2 access the same database items.

$$\begin{array}{|l|l|} \hline T1 & T2 \\ \hline \text{read(X)} & {} \\ X=X-N & {} \\ \hline {} & \text{read(X)} \\ {} & X=X+N \\ \hline \text{write(X)} & {} \\ \hline {} & \text{write (X)} \\ \hline \end{array}$$

Which one of the following statements is correct about these two transactions?

• A. Dirty Read will occur
• B. Lost update will occur
• C. Both T1 and T2 are serial schedule
• D. This is an example of uncommitted data problem

Answer 1

First T1 updates the value of X and then T2 overwrites the value of X written by T1. So the update made by T1 is lost. Hence, a lost update problem. Option B.

Comments

Should not it also be a dirty read problem? Since T2 reads uncommitted value of X?

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No.because T1 has not yet done the Write operation. So the value being read by T2 is not 'dirty'.

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Someone please verify shraddha’s comment. I have the same doubt as abhishek.

Answer 2

$X=X-N$

This is update, not write.

Write is specifically mentioned as write(X).

So, A is incorrect and B is correct.